The invention relates to a method for adjusting the black level of a video signal, in particular for television, which signal is transferred by a capacitor, whereby a conductor which conducts this signal is connected to a voltage source for a fraction of the duration of a picture scanning line.
It relates equally to a device for adjusting the black level of a video signal, in particular in a picture receiver.
Circuits capable of delivering the video signals with a continuous level of polarity deviating little with temperature, constant in time and relatively insensitive to supply voltage variations are expensive and complex. It has therefore long been the practice to transmit the video signal without its continuous polarity and to restore it subsequently. A circuit for restoring this polarity, that is to say for "adjusting the black level", is known from the work "guide de la television en couleurs" (Guide to color television) published by CHIRON (PARIS), page 120 and in particular figure IV.10.
This circuit is provided with a voltage source constituted by a set of diodes, some of which deliver a current in one direction and others which deliver a current in the other direction. This set of diodes is controllable by signals applied to certain of the diodes, and is linked with a connection which conducts the video signal.
This set of diodes constitutes a switch for linking the connection to a voltage source, namely ground, for a fraction of the duration of a picture scanning line.
Unfortunately the signal voltage level, the "pedestal", during the line retrace or flyback, is interrupted by a synchronization pulse whose level is different from that of the pedestal and which upsets the adjustment of the black level, that is to say of the pedestal. For this reason, control of the diodes has to take place solely during a strictly defined period of time and outside the synchronization pulse. To this end, the work cited prescribes the use of a color reference signal situated after the synchronization pulse. Unfortunately, no such signal exists in monochrome television sets. In this case it is necessary to generate an appropriate signal, for example by logically combining a line flyback (blanking signal) and the synchronization pulse. This necessitates the use of specific circuits and, in particular, of additional connections for taking off the signals ad hoc at different points in the receiver.